Catalyst-attached muffler for internal combustion engine

ABSTRACT

A catalyst-attached muffler includes an expansion chamber ( 25 ) defined by a cup-shaped fore-side outer wall panel ( 21 ) and a cup-shaped rear-side outer wall panel ( 22 ), both panels being removably attached to each other; a partition panel ( 23 ) interposed between the fore-side outer wall panel and the rear-side outer wall panel to partition the expansion chamber into a first expansion chamber ( 25 A) and a second expansion chamber ( 25 B). A closed-end cylindrical diffuser ( 40 ) is positioned in the first expansion chamber; and a gas-permeable exhaust gas-purifying catalyst ( 30 ) is inserted into and fixed to the diffuser, wherein exhaust gas from the exhaust port ( 10 ) of a cylinder of internal combustion engine is permitted to enter, via the catalyst, into the first expansion chamber, then to flow out of the first expansion chamber and to enter, via an opening ( 23   e ) formed in the partition panel, into the second expansion chamber, and to flow out of the second expansion chamber through an outlet ( 29 ) formed in the fore-side outer wall panel, thereby discharging the exhaust gas into external atmosphere.

FIELD OF THE INVENTION

The present invention relates to a muffler which includes an expansion chamber and an exhaust gas-purifying catalyst. In particular, the present invention relates to a catalyst-attached muffler which is suited for use in a small air-cooled two-stroke gasoline engine to be employed in a portable power working machine such as a brush cutter or a chain saw.

BACKGROUND INFORMATION

In recent years, reducing the quantity of hydrocarbon (HC), carbon monoxide (CO), etc. in exhaust gas from internal combustion engines has become quite important, even in an internal combustion engine such as a small air-cooled two-stroke gasoline engine to be employed in a portable power working machine such as a brush cutter or a chain saw, just as in the case of four-stroke automobile engines. Accordingly, it has been proposed to mount an exhaust gas-purifying catalyst inside the muffler installed on the exhaust gas system of the engine so as to enable the exhaust gas to pass through the catalyst, thereby purifying (reducing HC, CO, etc.) the exhaust gas.

JP Laid-open Patent Publication (Kokai) No. 2001-317325 (2001) shows one such catalyst-attached muffler, which is featured in that the expansion chamber of muffler is partitioned by a partition panel into a first expansion chamber and a second expansion chamber, that an exhaust gas-purifying catalyst is mounted on the partition panel enabling the catalyst to protrude into the first expansion chamber as well as into the second expansion chamber, and that the exhaust gas discharged from the exhaust port of the internal combustion engine is at first injected into the first expansion chamber and then discharged from the first chamber into the second expansion chamber through the catalyst, ultimately enabling the exhaust gas to be discharged into the external atmosphere from the outlet provided at the second expansion chamber.

Further, as seen from International Patent Publication No. 9-507898 (1997), the mounting of the muffler on the engine (the cylinder of engine) has been conventionally performed using relatively long bolts having a sufficient length to penetrate the muffler, the bolts being applied thereto from the outer side of the muffler (the outer wall panel of the muffler).

Additionally, it has recently been proposed to separately prepare a catalyst carrier (diffuser) and to secure the catalyst to this diffuser instead of directly mounting the catalyst on the partition panel.

The catalyst-attached muffler which is provided with such a diffuser will be explained hereinafter with reference to FIG. 9. The internal combustion engine 1′ shown in FIG. 9 is a small air-cooled two-stroke gasoline engine of the Schnürle type scavenging system which can be mounted as a power source on a portable power working machine such as a brush cutter or a chain saw. The displacement of the engine 1′ may be about 23 to 45 mL for example. This engine 1′ includes a cylinder 2′ having a domed combustion chamber 5′. The cylinder 2′ is provided with a large number of cooling fins 6′ on the outer circumference thereof. Moreover, the cylinder 2′ is provided with an exhaust port 10′ (for example, oval in cross-section) which is tapered in configuration, i.e., the cross-sectional area of the exhaust port 10′ being gradually increased from the inner end to the outer end thereof. Although not shown in the drawing, the cylinder 2′ is further provided with an air-intake port and a scavenging port, both ports being disposed in the known manner.

On the outer side of the exhaust port 10′ of the cylinder 2′, a muffler 20′ is installed. This muffler 20′ includes a couple of outer wall panels 21′ and 22′, i.e., a fore-side outer wall panel 21′ and a rear-side outer wall panel 22′, which are hermetically jointed together by a flat lock seam, thereby defining an expansion chamber 25′. The fore-side outer wall panel 21′ located on the exhaust port 10′ side is provided with an opening 21 a′ having approximately the same configuration as the outer end portion of the exhaust port 10′. A reinforcing plate 28′ is secured along the opening 21 a′ by welding, etc. Further, the rear-side outer wall panel 22′ is provided with a recessed portion 22 a′ which is located approximately coaxially with the exhaust port 10′. A closed-end square-tubular diffuser (catalyst holder) 40′ is interposed between the recessed portion 22 a′ and the reinforcing plate 28′ so as to interconnect the recessed portion 22 a′ with the reinforcing plate 28′. A heat-insulating/shock-absorbing plate 27′ is interposed between the catalyst holder 40′ and the recessed portion 22 a′.

At an inner end portion 41′ (on the exhaust port 10′ side) of the diffuser 40′ is a mounting hole 40 a′ for fitting and securing (by welding, etc.) a short cylindrical exhaust gas-purifying catalyst 30′. At an outer end portion 42′ (on the recessed portion 22 a′ side) of the diffuser 40′ is a predetermined number of exhaust gas-passing holes 43. The catalyst 30′ is formed of an oxidation catalyst comprising a gas-permeable metallic carrier and enclosed by a cylindrical shell 32′. This catalyst 30′ is held by the diffuser 40′ in such a manner that the central axis “O” of catalyst 30′ is approximately aligned with the central line of the exhaust port 10′ (i.e., the outer end portion 10 a′ thereof) and also with the central line of the diffuser 40′.

Further, the rear-side outer wall panel 22′ includes, at a lower portion thereof, an outlet port 29′ for discharging exhaust gas out of the muffler 20′. Although not shown in FIG. 9, a metallic spark arrester screen (SAM screen) and an exhaust gas-guiding member for regulating the discharge direction of exhaust gas, etc. may be mounted at this outlet port 29′.

In the muffler constructed as described above, the exhaust gas to be blown out of the exhaust port 10′ at nearly the velocity of sound is guided, through the inner end portion 41′ of the diffuser 40′ positioned in the muffler 20′, to the catalyst 30′ and forced to pass through the interior of the catalyst 30′ as shown by a solid arrow in FIG. 9. As the exhaust gas passes through the catalyst 30′, the quantities of HC, CO, etc. in the exhaust gas are reduced by the catalyst 30′. The exhaust gas that has passed through the catalyst 30′ is blown into the expansion chamber 25′ through the exhaust gas-passing holes 43′ of the diffuser 40′ and forced to expand and diffuse. As a result, the exhaust sound can be attenuated. The exhaust gas thus expanded and diffused in the expansion chamber 25′ is then permitted to move downward and be discharged out of the muffler 20′ through the outlet port 29′.

In the case of the aforementioned conventional catalyst-attached muffler, however, the processes of welding and copper brazing are chiefly employed for the retention and fixing of the components thereof such as the fore- and rear-side outer wall panels defining the expansion chamber, the partition panel, the diffuser, and the catalyst. This results in a higher manufacturing cost of the muffler.

Further, since the mounting of the muffler on the cylinder of the engine has been conventionally performed using relatively long bolts, the bolts being applied thereto from the outer side of the muffler (the outer wall panel of the muffler), the location of fastening or fixing the muffler is fairly distanced away from the cylinder. As a result, the stability and rigidity in the attachment of the muffler to the cylinder are more likely to be deteriorated. Furthermore, since it is required to provide the outer wall panel with holes for inserting the bolts, the structure including the outer wall panel would become complicated, thus raising another problem.

Furthermore, in the case of the muffler provided with a diffuser as shown in FIG. 9, it is required to provide the diffuser with mounting holes for sustaining and securing the catalyst to the diffuser, and thus the construction of the muffler would become more complicated as a whole. Additionally, as shown in FIG. 10, since the cross-sectional configuration of the catalyst 30′ differs from the shape of the opening of the exhaust port 10′ and from the cross-sectional configuration of the diffuser 40′ (i.e., the inner end portion 41′ thereof), most of the exhaust gas is caused to pass through a central portion of the catalyst 30′, so that the catalyst 30′ existing at the peripheral portion thereof is substantially kept away from the flow of exhaust gas, thus raising the problem that the purifying capacity of the catalyst 30′ cannot be sufficiently utilized.

Moreover, according to the aforementioned conventional muffler, since it is designed such that the spark arrester screen and the exhaust gas-guiding member are adapted to be fastened to the outlet 29′ of the rear-side outer wall panel 22′ by machine screws, the number of parts (screws and the like) would be increased and at the same time, the position and configuration of the outlet 29′ would be restricted, whereby the design freedom of the muffler would be adversely affected.

The present invention has been made in view of overcoming the aforementioned problems, and therefore an object of the present invention is to provide a catalyst-attached muffler for internal combustion engine, which is capable of saving the manufacturing cost of the muffler as much as possible by reducing the number of welding portions in the manufacture of the muffler, capable of enhancing the stability and rigidity in the attachment of the muffler to the cylinder by directly securing the muffler to the cylinder without necessitating the employment of long bolts, capable of utilizing the purifying capacity of the catalyst to a greater extent, and capable of increasing the design freedom with respect to the position and configuration of the outlet for the exhaust gas.

SUMMARY OF THE INVENTION

With a view to realize the aforementioned object, the catalyst-attached muffler according to the present invention is basically constructed such that it comprises an expansion chamber which is defined by a cup-shaped fore-side outer wall panel and by a cup-shaped rear-side outer wall panel, both panels being removably attached to each other; a partition panel interposed between the fore-side outer wall panel and the rear-side outer wall panel to partition the expansion chamber into a first expansion chamber and a second expansion chamber; a closed-end cylindrical diffuser positioned in the first expansion chamber; and a gas-permeable exhaust gas-purifying catalyst inserted into and fixed to the diffuser; wherein exhaust gas blown out of the exhaust port formed in a cylinder of an internal combustion engine is permitted to enter, via the catalyst, into the first expansion chamber, then to flow out of the first expansion chamber and to enter, via an opening formed in the partition panel, into the second expansion chamber, and to flow out of the second expansion chamber through an outlet formed in the fore-side outer wall panel, thereby discharging the exhaust gas into the external atmosphere.

In an exemplary embodiment, a fore-end portion of the fore-side outer wall panel, a fore-end portion of the partition panel, and a flange portion formed at fore-end of the diffuser are collectively secured to a brim portion of the exhaust port of the cylinder by bolts.

In an exemplary embodiment, the catalyst is cylindrical or prismatic in configuration and fitted in and fixed, by spot welding, to the diffuser.

Preferably, the catalyst is an oxidation catalyst formed of metallic carrier.

In an exemplary embodiment, a spark arrester screen is attached to the opening formed in the partition panel.

In another exemplary embodiment, a rear-end portion of the fore-side outer wall panel, a rear-end portion of the partition panel, and a fore-end portion of the rear-side outer wall panel are respectively formed into a collar-shaped portion and these collar-shaped portions are successively superimposed and collectively fastened together by bolts.

According to the catalyst-attached muffler of the present invention, since the expansion chamber is defined by making use of a cup-shaped fore-side outer wall panel and a cup-shaped rear-side outer wall panel, both panels being separable from each other, the fore-side outer wall panel can be kept open prior to the attachment of the rear-side outer wall panel to the fore-side outer wall panel for defining the expansion chamber, so that anchoring bolts can be inserted into the fore-side outer wall panel to directly tighten it to the cylinder. Therefore, the anchoring bolts to be employed in this case may be relatively short in length and moreover the fore-side outer wall panel, the partition panel and the diffuser can be collectively secured to the cylinder by making use of such short bolts. As a result, as compared with the prior art wherein long bolts are used for fastening or fixing the muffler to the cylinder, the muffler can be fastened to the cylinder at a location which is much closer to the cylinder, thereby making it possible to enhance the stability and rigidity in the attachment of the muffler to the cylinder.

Further, not only because all of the fore-side outer wall panel, the partition panel and the diffuser can be collectively secured to the cylinder, but also because the catalyst is fitted in (press-fitting) the diffuser and fixed to the diffuser by spot welding, it is possible to reduce, as compared with the prior art, the operation of welding which is rather troublesome, resulting in the saving of manufacturing costs.

Furthermore, since the catalyst for cleaning exhaust gas is cylindrical or prismatic in configuration and fitted in the diffuser, it possible to make the entire region of the catalyst available for the cleaning of the exhaust gas. As a result, it is now possible to utilize the purifying capacity of the catalyst to a greater extent and hence to greatly enhance the exhaust gas purifying performance of the catalyst.

Moreover, since it is designed such that, in addition to the outlet for exhaust gas which is provided in the fore-side outer wall panel, an opening is separately formed in the partition panel and the spark arrester screen is attached to this opening, the design freedom regarding the position and configuration of the outlet for exhaust gas can be considerably increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating one embodiment of the catalyst-attached muffler according to the present invention together with part of the internal combustion engine to which this muffler is applied;

FIG. 2 is a plan view illustrating the diffuser shown in FIG. 1;

FIG. 3 is an enlarged cross-sectional view taken along the line III-III in FIG. 2, illustrating a brim portion of the exhaust port as well as part of diffuser;

FIG. 4 is a plan view showing the fore-side outer wall panel shown in FIG. 1;

FIG. 5 is a cross-sectional view taken along the line V-V in FIG. 4;

FIG. 6 is a plan view showing the partition panel shown in FIG. 1;

FIG. 7 is a cross-sectional view of part of FIG. 1, illustrating fore-side and rear-side outer wall panels and a state wherein these outer wall panels and the partition panel are collectively fastened;

FIG. 8 is a cross-sectional view schematically illustrating a modified example of the catalyst-attached muffler shown in FIG. 1;

FIG. 9 is a cross-sectional view schematically illustrating one example of the conventional catalyst-attached muffler together with part of the internal combustion engine to which this muffler is applied; and

FIG. 10 is a cross-sectional view taken along the line X-X in FIG. 9.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT

Next, one embodiment of the muffler according to the present invention will be explained with reference to the drawings.

FIG. 1 shows a cross-sectional view schematically illustrating one embodiment of the catalyst-attached muffler according to the present invention together with part of the internal combustion engine to which this muffler is applied.

Just like the conventional internal combustion engine shown in FIG. 9, an internal combustion engine 1 shown in FIG. 1 is a small air-cooled two-stroke gasoline engine of Schnürle type scavenging system which can be mounted as a power source on a portable power working machine such as a brush cutter or a chain saw. The displacement of the engine 1 may be about 23 to 45 mL for example. This engine 1 includes a cylinder 2 having a domed combustion chamber 5. The cylinder 2 is provided with a large number of cooling fins 6 on the outer circumference thereof. Moreover, the cylinder 2 is provided with an exhaust port 10 which is circular or oval in cross-section and generally tapered in configuration (bell-shaped), i.e., the cross-sectional area of the exhaust port 10 is gradually increased from the inner end to the outer end thereof. Although not shown in the drawing, the cylinder 2 is further provided with an air-intake port and a scavenging port, both ports being disposed in the known manner.

On the outer side of the exhaust port 10 of the cylinder 2, a muffler 20 is attached. This muffler 20 includes an expansion chamber 25 defined by a cup-shaped fore-side outer wall panel 21 and a cup-shaped rear-side outer wall panel 22, which are configured separable from each other. The expansion chamber 25 is partitioned into a first expansion chamber 25A and a second expansion chamber 25B by a cup-shaped partition panel 23 which is positioned inside the fore-side outer wall panel 21.

The fore-side outer wall panel 21 is provided, in the vicinity of a central portion of the fore-end portion 21A thereof which is located on the exhaust port 10 side, with an opening 21 a having approximately the same size and configuration as those of the outer end portion of the exhaust port 10. A rhomboidal (in plan view) reinforcing plate 28 having rounded corners and an opening 28 a of the same size and configuration as the opening 21 a is secured along the periphery of this opening 21 a. (see also FIG. 4). The reinforcing plate 28 is fixed to the opening 21 a by spot welding.

The fore-side outer wall panel 21 is provided, at a rear end thereof, with a collar-shaped portion 21 b having an L-shaped cross-section. This collar-shaped portion 21 b is provided, at four corners thereof, with bolt-holes 21 c as shown in FIG. 4.

The rear-side outer wall panel 22 is provided, at a fore end thereof, with a collar-shaped portion 22 b having an L-shaped cross-section which is adapted to be engaged with the inner side of the collar-shaped portion 21 b of the fore-side outer wall panel 21. This collar-shaped portion 22 b is also provided, at four corners thereof, with bolt-holes 22 c which are designed to be aligned with the bolt-holes 21 c of the fore-side outer wall panel 21 (see FIG. 7).

The partition panel 23 is provided, in the vicinity of a central portion of the fore-end portion 23A thereof, with a circular opening 23 a and also provided, at a rear end thereof, with a collar-shaped portion 23 b which is adapted to be sandwiched between the collar-shaped portion 21 b of the fore-side outer wall panel 21 and the collar-shaped portion 22 b of the rear-side outer wall panel 22. This collar-shaped portion 23 b is also provided, at four corners thereof, with bolt-holes 23 c which are designed to be aligned with the bolt-holes 21 c of the fore-side outer wall panel 21 as well as with the bolt-holes 22 c of the rear-side outer wall panel 22.

The fore-end portion 21A (the outer surface thereof) of the fore-side outer wall panel 21 contacts a projected portion 2A where the exhaust port 10 of the cylinder 2 is. The fore-end portion 23A (the outer surface thereof) of the partition panel 23 contacts the reinforcing plate 28. At a proximal end of the closed-end cylindrical diffuser 40, a rhomboidal (in plan view) collar-shaped portion 41 with rounded corners, as shown in FIG. 2, contacts the fore-end portion 23A (the inner surface thereof) of the partition panel 23 so as to hermetically seal the periphery of the circular opening 23 a of fore-end portion 23A of the partition panel 23. In this case, the fore-side outer wall panel 21 (the reinforcing plate 28), the partition panel 23, and the diffuser 40 are arranged in such a manner that the central lines of the aforementioned openings 21 a, 28 a and 23 a are all aligned with the central axis “O” of the diffuser 40.

As shown in FIGS. 2, 3, 4 and 6, the fore-end portion 21A of the fore-side outer wall panel 21, the reinforcing plate 28, the fore-end portion 23A of the partition panel 23, and the collar-shaped portion 41 of the diffuser 40 are provided with a couple of bolt holes 21 d, a couple of bolt holes 28 d, a couple of bolt holes 23 d, and a couple of bolt holes 41 d, respectively. Each of these couples of bolt holes are positioned in place so as to align with each other. Therefore, bolts 26 may be inserted into all of these bolt holes 21 d, 28 d, 23 d and 41 d and screwed into the projected portion 2A of the cylinder 2, thereby making it possible, through the employment of the bolts 26, to directly and collectively fix all of the fore-side outer wall panel 21 (the reinforcing plate 28), the partition panel 23 and the diffuser 40 to the cylinder 2.

Further, as shown in FIG. 7, the collar-shaped portion 21 b of the fore-side outer wall panel 21, the collar-shaped portion 23 b of the partition panel 23, and the collar-shaped portion 22 b of the rear-side outer wall panel 22 are successively superimposed and collectively clamped together in a hermetical manner by making use of bolts 37 which are successively inserted into the bolt holes 21 c, 23 c and 22 c and screwed on nuts 38 which are welded in advance to the collar-shaped portion 21 b of the fore-side outer wall panel 21. Therefore, when the clamping using the bolts 37 is loosened, the rear-side outer wall panel 22 can be easily separated from the fore-side outer wall panel 21, thus facilitating the maintenance and inspection of the muffler for example.

In the fore-end portion 40A of the diffuser 40, there is fitted (press-fitting) a cylindrical exhaust gas-purifying catalyst 30 which is secured thereto by spot welding (spot welded portions 44). A couple of exhaust gas-passing holes 43 which are located diametrally opposite to each other are provided on the circumferential surface of the rear-end portion 40B of the diffuser 40. The catalyst 30 is formed of an oxidation catalyst comprising a gas-permeable metallic carrier and enclosed by a cylindrical metallic shell 32.

Further, as shown in FIGS. 4 and 5, the fore-side outer wall panel 21 is provided, on one side of the rear-end portion thereof, with a louvered outlet port 29 which is embossed inward. As shown in FIGS. 1 and 6, on one side of the rear-end portion of the partition panel 23, which is located diametrally opposite to the louvered outlet port 29, is a recessed surface 23D which is recessed inward. This recessed surface 23D is provided with a rectangular opening 23 e. A spark arrester screen 35 is mounted on the recessed surface 23D so as to cover this opening 23 e by any suitable method such as spot welding. Although not shown in the drawing, an exhaust gas guiding member for regulating the discharging direction of exhaust gas is mounted at this louvered outlet port 29.

In the muffler constructed as described above, the exhaust gas to be blown out of the exhaust port 10 at nearly the velocity of sound is guided to the diffuser 40 through the opening 21 a of the fore-side outer wall panel 21, the opening 28 a of the reinforcing plate 28 and the opening 23 a of the partition panel 23 as shown by a solid arrow in FIG. 1, thus enabling the exhaust gas to pass through the interior of the catalyst 30 which is fitted in and secured to the diffuser 40. As the exhaust gas passes through the catalyst 30, the quantities of HC, CO, etc. in the exhaust gas are reduced by the catalyst 30. The exhaust gas that has passed through the catalyst 30 is blown into the first expansion chamber 25A through the exhaust gas-discharging holes 43 of the diffuser 40 and forced to expand and diffuse. As a result, the exhaust sound can be attenuated. The exhaust gas thus expanded and diffused in the first expansion chamber 25A is then introduced, through the spark arrester screen 35, into the second expansion chamber 25B and thereafter discharged out of the second expansion chamber 25B through the outlet port 29 formed in the fore-side outer wall panel 21.

According to the catalyst-attached muffler 20 of this embodiment, since the expansion chamber 25 is defined by making use of a cup-shaped fore-side outer wall panel 21 and a cup-shaped rear-side outer wall panel 22, both panels being separable from each other, the fore-side outer wall panel 21 can be kept open prior to the attachment of the rear-side outer wall panel 22 to the fore-side outer wall panel 21 for defining the expansion chamber, so that anchoring bolts 26 can be easily inserted into the fore-side outer wall panel 21 to directly tighten it to the cylinder 2. Therefore, the anchoring bolts 26 to be employed in this case may be relatively short in length and moreover the fore-side outer wall panel 21, the partition panel 23 and the diffuser 40 can be collectively secured to the cylinder 2 by making use of such short bolts 26. As a result, as compared with the prior art wherein long bolts are used for fastening or fixing the muffler to the cylinder 2, the muffler can be fastened to the cylinder 2 at a location which is much closer to the cylinder 2, thereby making it possible to enhance the stability and rigidity in the attachment of the muffler to the cylinder 2.

Further, not only because all of the fore-side outer wall panel 21, the partition panel 23 and the diffuser 40 can be collectively secured to the cylinder, but also because the catalyst 30 is fitted in (press-fitting) the diffuser 40 and fixed to the diffuser 40 by spot welding, it is possible to reduce, as compared with the prior art, the operation of welding which is rather troublesome, resulting in the saving of manufacturing costs.

Furthermore, since the catalyst 30 for purifying exhaust gas is cylindrical in configuration and fitted in the closed-end cylindrical diffuser 40 which is capable of receiving all of the exhaust gas to be discharged from the exhaust port 10, it possible to make the entire region of the catalyst available for the purifying of the exhaust gas. As a result, it is now possible to utilize the purifying capacity of the catalyst 30 to a greater extent and hence to greatly enhance the exhaust gas purifying performance of the catalyst 30.

Moreover, since it is designed such that, in addition to the outlet 29 for exhaust gas which is provided in the fore-side outer wall panel 21, an opening 23 e is separately formed in the partition panel 23 and the spark arrester screen 35 is attached to this opening 23 e, the design freedom regarding the position and configuration of the outlet 29 for exhaust gas can be considerably increased.

While, in the foregoing description, one embodiment of the present invention has been explained in details for the purpose of illustration, it will be understood that the construction of the device can be variously modified without departing from the spirit and scope of the invention claimed in claims attached herewith.

For example, although the spark arrester screen 35 is mounted on the recessed surface 23D in the previous embodiment, the location of mounting the spark arrester screen 35 may not be limited to the recessed surface 23D but may be the upper surface 23B of the partition panel 23 for example as shown in FIG. 8.

Further, in the previous embodiment, although a couple of exhaust gas-passing holes 43 are formed on the circumferential surface of the rear-end portion 40B of the closed-end cylindrical diffuser 40 so as to be located diametrally opposite to each other, these exhaust gas-passing holes 43 may be provided at the bottom 40C of the diffuser 40 as shown in FIG. 8.

In order to prevent the surface of the outer wall panel 22 from becoming excessively high in temperature, an inner wall panel 36 may be provided on the inner side of the outer wall panel 22 so as to form a heat insulating space 39 between the outer wall panel 22 and the inner wall panel 36. In this case, in the same manner as in the cases of the collar-shaped portions 21 b, 23 b and 22 b of the fore-side outer wall panel 21, the partition panel 23 and the rear-side outer wall panel 22, a collar-shaped portion 36 b may be formed at the fore-end of the inner wall panel 36 and interposed between the collar-shaped portion 23 b of the partition panel 23 and the collar-shaped portion 22 b of the rear-side outer wall panel 22 so as to collectively fix it thereto.

Further, the catalyst-attached muffler of the present invention can be also applied likewise to a four-stroke engine in addition to the air-cooled two-stroke gasoline engine which is employed in the aforementioned embodiment.

Further, with respect to the exhaust gas-cleaning catalyst, it is also possible to employ a catalyst in the form of a foamed molded body or a ceramic carrier, in addition to the aforementioned embodiment (oxidation catalyst formed of metallic carrier). 

1. A muffler capable of cooperating with an exhaust port of a cylinder of an internal combustion engine, the muffler comprising: an expansion chamber, the expansion chamber including a cup-shaped fore-side outer wall panel and a rear-side outer wall panel, both panels being removably secured to each other; a partition panel interposed between the fore-side outer wall panel and the rear-side outer wall panel to partition the expansion chamber into a first expansion chamber and a second expansion chamber; a diffuser positioned in the first expansion chamber; and a catalyst inserted into and secured to the diffuser; wherein exhaust gas from the exhaust port flows through the catalyst and is introduced into the first expansion chamber, then flows out of the first expansion chamber and is introduced into the second expansion chamber via an opening formed in the partition panel, and flows out of the second expansion chamber through an outlet formed in the fore-side outer wall panel.
 2. The muffler of claim 1, wherein a fore-end portion of the fore-side outer wall panel, a fore-end portion of the partition panel, and a flange portion formed at the fore-end of the diffuser are collectively secured to a portion of the exhaust port of the cylinder by bolts.
 3. The muffler of claim 2, wherein the head of at least one of the bolts is proximate to the portion of the exhaust port.
 4. The muffler of claim 1, wherein the catalyst is at least one of a cylindrical configuration and a prismatic configuration.
 5. The muffler of claim 4, wherein the catalyst is secured to the diffuser by spot welding.
 6. The muffler of claim 1, wherein the catalyst is secured to the diffuser by spot welding.
 7. The muffler of claim 1, wherein the catalyst is an oxidation catalyst formed of metallic carrier.
 8. The muffler of claim 1, further comprising a spark arrester screen secured to the opening formed in the partition panel.
 9. The muffler of claim 1, wherein a rear-end portion of the fore-side outer wall panel, a rear-end portion of the partition panel, and a fore-end portion of the rear-side outer wall panel are respectively formed into a first, second, and third collar-shaped portion and the first, second, and third collar-shaped portions are successively superimposed and collectively fastened together by bolts.
 10. A muffler capable of cooperating with an exhaust port of a cylinder of an internal combustion engine, the muffler comprising: an expansion chamber, the expansion chamber including a cup-shaped fore-side outer wall panel and a cup-shaped rear-side outer wall panel, both panels being removably secured to each other; a partition panel interposed between the fore-side outer wall panel and the rear-side outer wall panel to partition the expansion chamber into a first expansion chamber and a second expansion chamber; a closed-end cylindrical diffuser positioned in the first expansion chamber; and a gas-permeable exhaust gas-purifying catalyst inserted into and secured to the diffuser; wherein exhaust gas from the exhaust port flows through the catalyst and is introduced into the first expansion chamber, then flows out of the first expansion chamber and is introduced into the second expansion chamber via an opening formed in the partition panel, and flows out of the second expansion chamber through an outlet formed in the fore-side outer wall panel.
 11. The muffler of claim 10, wherein a fore-end portion of the fore-side outer wall panel, a fore-end portion of the partition panel, and a flange portion formed at the fore-end of the diffuser are collectively secured to a brim portion of the exhaust port of the cylinder by bolts.
 12. The muffler of claim 11, wherein the head of at least one of the bolts is proximate to the brim portion of the exhaust port.
 13. The muffler of claim 10, wherein the catalyst has a cylindrical configuration and a prismatic configuration.
 14. The muffler of claim 13, wherein the catalyst is secured to the diffuser by spot welding.
 15. The muffler of claim 10, wherein the catalyst has a prismatic configuration.
 16. The muffler of claim 15, wherein the catalyst is secured to the diffuser by spot welding.
 17. The muffler of claim 10, wherein the catalyst is secured to the diffuser by spot welding.
 18. The muffler of claim 10, wherein the catalyst is an oxidation catalyst formed of metallic carrier.
 19. The muffler of claim 10, further comprising a spark arrester screen secured to the opening formed in the partition panel.
 20. The muffler of claim 10, wherein a rear-end portion of the fore-side outer wall panel, a rear-end portion of the partition panel, and a fore-end portion of the rear-side outer wall panel are respectively formed into a first, second, and third collar-shaped portion and the first, second, and third collar-shaped portions are successively superimposed and collectively fastened together by bolts. 